Particularly in connection with the production of multicoat paint systems for automotive production-line (OEM) finishing, inorganic layered composites, such as, for example, clay minerals, such as, more particularly, naturally occurring smectite types, such as montmorillonite, saponite or hectorite, for example, or synthetic smectite types, such as laponites, for example, are of great importance for adjusting the rheology of the color-imparting aqueous basecoat materials. The aforementioned minerals have a negative surface charge and a positive edge charge, which positively influences the adjustment of the rheology of the basecoat materials, particularly in the case of those applications where there is a high shearing stress on the basecoat materials. However, aqueous coating materials comprising the aforementioned minerals have a tendency toward gelling, which may impair the processing properties of the aqueous basecoat materials or, where they have good processing properties, limits the level of nonvolatile material in the aqueous basecoat material.
Where there are effect pigments, especially anisotropic effect pigments, in the aqueous basecoat materials, such as aluminum flakes or mica pigments, for example, then the use of the minerals referred to above is an issue, since in the baked coating films there are often impairments of the metallic effect, particularly with regard to the flop behavior, and also with regard to luminance and clouding, in other words in connection with the development of areas of light/dark shading.
WO-A-02/053658 describes laponite-containing aqueous basecoat materials which comprise effect pigment (referred to below as aqueous effect basecoat materials) which for the purpose of preventing clouding comprise a mixture of at least two neutralized fatty acids. Although the properties of the aqueous effect basecoat materials WO-A-02/053658 describes are already excellent, there is a need, particularly in the case of aqueous metallic effect basecoat materials, to obtain an optimum combination of good rheological properties and outstanding optical properties with high fractions of nonvolatile material (solids). Particularly important in this respect is the combination of an outstanding metallic effect (flop) with a high luminance (high fraction of reflected light and low fraction of diffusely scattered light).
WO-A-2007/065861 describes hydroxides, more particularly hydrotalcite types, which contain at least 2 organic anions with at least 8 carbon atoms as counterions, it being possible for the anions to contain further functional groups, such as hydroxyl, amino or epoxide groups, for example. The hydrophobic hydrotalcites thus modified are described for use as intercalatable fillers for polymers, especially for rubberlike polymers. The use of hydrotalcites in coating compositions is described in general terms in WO-A-2007/065861. The hydrophobic hydrotalcites are of only limited suitability for use in aqueous coating compositions for OEM coat systems, since their compatibility with the preferably water-dispersible binders at the molecular level is poor. The use of the coating compositions for improving the optical properties, in particular the production of an outstanding metallic effect (flop) having a high luminance in the case of aqueous effect basecoat materials, is not described in WO-A-2007/065861.
In the light of the prior art, the problem addressed by the present invention is that of providing aqueous effect basecoat materials having a high fraction of nonvolatile constituents, which materials, in particular as basecoat film in a coating system in production-line automobile finishing, have an outstanding metallic effect (flop) having a high luminance.